1. Technical Field
This invention relates to removal of carbon from particulate mixtures by catalyzed oxidation. All elemental forms of carbon, other than diamonds, can be removed from the particulate mixture using the process. The process is especially useful for removal of residual carbon in the manufacture of synthetic diamonds.
2. Description of Related Art
Carbon can be removed from some particulate mixtures by conventional physical procedures such as settling, filtration, use of centrifuges, and distillation. Physical separation techniques cannot be used, however, if the carbon particles are too small, because of tight bonding between the carbon particles and other particulates in the mixture. Chemical procedures, such as air oxidation, must be used in these situations.
If the other mixture components are highly resistant to oxidation (such as many inorganic oxides, silicates, and clays), carbon can be removed by heating the mixture above 600.degree. C. in an oxidizing atmosphere, without significant loss or modification of other mixture components. Fixed-bed, fluidized bed, and rotary kilns may be used for this purpose. Catalysts may be used to reduce the temperature needed to oxidize the carbon if the mixture contains valuable components, which are to be recovered, that are unstable at higher temperatures.
U.S. Pat. No. 3,348,918 describes a process for removing carbon from carbon/diamond mixtures by selective oxidation with molecular oxygen in the presence of a lead oxide catalyst at a temperature within the range of about from 250 to 500.degree. C. The catalyst is uniformly distributed throughout the carbon/diamond mixture. For example, an oxide or an oxygen containing compound of lead which is transformed in situ to an oxide of lead at the reaction temperature or below, e.g., lead carbonate, hydroxide, nitrate, subacetate, etc., is homogeneously mixed with the powder mixture either by hand or mechanically.
E. Ger. Pat. No. DD 224,575 A1 describes a process for removing carbon from carbon-diamond mixtures wherein an aqueous solution of copper salts, which decompose at temperatures higher than 450.degree. C. with the formation of CuO or Cu.sub.2 O, is mixed with the carbon-diamond mixture. The suspension is dried and reacted at a maximum temperature of 540.degree. C. with addition of oxygen containing gases. The reaction product is boiled with hydrochloric or nitric acid, the acid is washed out with water and the product dried. X-ray analysis indicates that the residue is an almost graphite-free diamonds.
Unreacted carbon must be removed in the manufacture of diamonds by explosive shock synthesis. The current industrial practice is to mix a catalytic amount of lead oxide powder with the carbon/diamond material produced by explosive shock synthesis, and to heat the mixture in air at about 400.degree. C. for several hours. The non-diamond carbon thereby is oxidized, forming gaseous CO and CO.sub.2 that is vented from the process, leaving a mixture of diamonds and lead oxide catalyst. Acid leaching removes the lead oxide catalyst; diamonds are recovered.
Industry is seeking to replace lead compounds in commercial processes for environmental and health reasons. Moreover, lead oxide may reduce the diamond yield in the process described above due to diamond "burn-up", which especially causes loss of valuable sub-micron particles.
Thus, there is a need for an improved process to remove carbon from valuable particulate material, such as diamond/carbon mixtures.